I. Field
The present invention relates generally to wireless communications, and more specifically to techniques for improving data transmission reliability.
II. Background
Many wireless communication systems employ HARQ to improve reliability for data transmission. With HARQ, each data packet may be transmitted once or multiple times by a transmitter until the packet is correctly decoded by a receiver or the maximum number of transmissions has been sent for the packet. A HARQ entity at the transmitter (which is often called a transmitter HARQ entity) receives packets that have been assigned sequence numbers, encodes each packet into one or several subpackets, and transmits these subpackets in sequential order.
A corresponding HARQ entity at the receiver (which is often called a receiver HARQ entity) receives the transmissions from the transmitter and combines the subpackets that belong to the same packet. The combined subpackets for each transmitted packet are then decoded in an attempt to recover the transmitted packet. However, due to degradation caused by deleterious effects in a wireless link, some of the received packets may be decoded in error and are called erased packets. The receiver may send to the transmitter an acknowledgment (ACK) for each correctly decoded packet to terminate the transmission of further subpackets for this packet and/or a negative acknowledgment (NAK) for each erased packet to initiate transmission of another subpacket for the packet. The transmitter may erroneously receive the ACKs and/or NAKs sent by the receiver. Each ACK that is erroneously detected as a NAK by the transmitter results in the transmission of another subpacket for a packet that has already been correctly decoded by the receiver. The redundant transmission may be correctly decoded by the receiver and result in a duplicate packet. The error rate for the ACK transmissions may be high and, consequently, the receiver may frequently obtain duplicate packets.
The receiver HARQ entity is also tasked with providing correctly decoded packets to upper layers. In many systems, the upper layers expect to receive data in the proper order, as determined by the sequence numbers of the packets. With HARQ, even though the transmitter HARQ entity sends the first subpackets in sequential order, the receiver HARQ entity may recover the packets out of order because of the additional subpacket transmissions for erased packets. As a result, the receiver HARQ entity typically buffers packets that have been correctly decoded, re-orders these packets as necessary, and provides the re-ordered packets to the upper layers. If packets are recovered out of order, then the receiver HARQ entity may “stall” or delay the delivery of correctly decoded packets to the upper layers whenever earlier packets are detected to be missing, until either (1) the missing packets are correctly decoded by the receiver HARQ entity or (2) the receiver HARQ entity is confident that the missing packets are lost and will not be received. If the receiver HARQ entity declares a packet to be lost when such is not the case, then the upper layers may (1) initiate retransmission of the lost packet, albeit with a long delay, or (2) treat the packet as being lost, both of which are undesirable.
In a simple re-ordering scheme, the receiver stores each correctly decoded packet in a buffer until the maximum time duration for transmitting all earlier packets has expired. The receiver then provides the correctly decoded packet to the upper layers after this maximum time duration has expired, with confidence that any earlier packets that are still missing will not be received. However, the delay for delivering packets to the upper layers may be overly long for this simple re-ordering scheme.
There is therefore a need in the art for techniques to efficiently perform duplicate detection and re-ordering for a HARQ transmission.